The present invention broadly relates to a method of fabricating a hollow body and, more specifically, pertains to a new and improved method of fabricating hollow bodies such as tubes, tubular torsion bars, pressure vessels and the like built-up from fiber-reinforced plastic.
Generally speaking, the method of the present invention is for fabricating a hollow body, especially tubes, tubular torsion bars, pressure vessels and the like built-up from fiber-reinforced plastic material, especially duroplastic or thermosetting plastic material, and having at least two layers of fibers, especially glass fibers, carbon fibers, aramide fibers and the like, mutually differently oriented in relation to a longitudinal axis of the hollow body. The fibers of the hollow body to be fabricated extend at least partially in each such differently oriented extension over the entire hollow body. The layers comprise a first plastic which at least at a foreseen service temperature is hard-elastic. One step of the method of the present invention entails subjecting the at least two layers to forces during a hard-elastic mutual bonding of the at least two layers to one another.
The fabrication of extremely light, yet highly structurally resistant, structural elements has long been strived for. This combination of properties can be obtained by employing fiber-reinforced plastics. These materials and the structural elements fabricated from them have primarily the advantage that very high strength or resistance to loading in the direction of fiber orientation can be achieved at low density. They have on the other hand the serious disadvantage of a very low strength or resistance to loading in a direction transverse to fiber orientation and an extremely low elongation at failure. It has long been attempted to overcome these deficiencies by employing more suitable plastics, especially plastics having increased elongation at failure. However, by selectively incorporating internal stresses or pre-stressing of a nature favorable in relation to the loading conditions, improvements can be obtained which amount to much more than the improvements obtainable by exchanging or modifying the plastic material.
A method of this type for fabricating plastic tubes or pipes is known from the German Pat. No. 2,842,531 in which a double-layered tube is built-up. First, an inner tube is fabricated having fibers extending essentially in the circumferential direction such that the fibers include an angle of essentially 90.degree. with the longitudinal direction or axis of the tube. Deviations from this orientation are only required to permit a continuous fiber to be wound or coiled around the tube in a slight lead or pitch angle. This tube is hardened or cured and a pressure medium is introduced into its interior while an axially effective compression force is applied to it simultaneously from the exterior. The plastic which bonds the individual fibers together is a hard-elastic plastic material and consequently an elastic deformation arises. A second tube layer containing glass fibers is applied to the thus elastically deformed body with the glass fibers oriented in essentially the axial direction. This second tube or tube layer, which is bound to the first tube, for instance by the hard-elastic plastic material, is also hardened, and the first tube or tube layer is subsequently relieved of the pressure medium and of the compressive force in the axial direction. During this relieving or unloading procedure, the inner tube can only partially relax since oppositely oriented tensile forces arise in the outer tube or tube layer during this procedure. The desired compressive stresses transverse to the direction of fiber orientation remain as internal stresses or prestressing at least in the inner tube or tube layer.
In soft-elastic or resilient tubes or hoses built-up from, for instance, resilient polyvinyl chloride, rubber or the like, it is known to provide reinforcement layers or inserts which assume an orientation of essentially about 55.degree. to the longitudinal axis. For instance, a woven mesh or fabric can be provided which exhibits equal angles of different sign.